Quantized electron accumulation layers on ZnO surfaces produced by low-energy hydrogen-ion implantation

Implantation of 100-eV hydrogen ions on the (0001̄) face of ZnO produces extremely strong electron accumulation layers on the surface. Calibrated argon-ion sputtering measurements indicate that the implanted ions (consisting mostly of protons) penetrate to a depth of 10-20 Å beneath the surface. They act as fully ionized donors giving rise to a free electron layer of comparable depth and of surface electron densities up to 2×10 ¹⁴ cm^-2. The narrow width, combined with detailed transport measurements, show that the implanted accumulation layers closely resemble ordinary accumulation layers on ZnO surfaces obtained, for example, by exposing the surface to atomic hydrogen. Thus the implanted layers, just as the ordinary accumulation layers, constitute a quantized, two-dimensional electron gas system. The important advantage of the implanted over the ordinary accumulation layers, however, is that they are completely inert to oxygen or even to room air. Annealing experiments provide estimates for the activation energies for release of the hydrogen ions from the surface layer. The results indicate the presence of several different types of lattice sites for the implanted ions.